In one aspect, the invention relates to a catalytic cracking unit. In another aspect, the invention relates to a catalytic cracking process.
In the petroleum refining industry, the fluidized catalytic cracking of hydrocarbons is well known and may be accomplished in a variety of processes which employ fluidized solid techniques. Normally in such processes, suitably preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely-divided, solid catalyst particles either in a fluidized bed reaction zone or in an elongated riser reaction zone, and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a portion, preferably a major portion, of the activity to the coke-contaminated or spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone. Typical regeneration zones comprise large vertical cylindrical vessels wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air, under conditions to burn at least a portion, preferably a major portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn from the regeneration zone and reintroduced into the reaction zone for reaction with additional hydrocarbon feed.
High boiling oils are difficult to catalytically crack to gasoline range product in existing catalytic cracking operations. There are several reasons for this. The deposition of large amounts of coke on the catalyst will frequently bring the unit up to its coke burning capacity. Coke precursors are more abundant in high boiling oils. Coke laydown is also caused by the deposition of metals on the cracking catalyst that increase the coking tendencies of the catalyst. The troublesome metals become concentrated in the high boiling oils. Coke laydown to a large extent is also influenced by poor vaporization of the oil prior to contact with the catalyst. High boiling oils are difficult to vaporize. Poor mixing between the cracking catalyst and oil feedstock also contributes to coke laydown on the catalyst, as poor mixing can lead to localized high catalyst:oil ratios and over cracking.
Heavy oils include heavy gas oils which generally boil from about 600.degree. F. to 1200.degree. F., and components such as topped crudes and residuum which frequently have an initial boiling point in excess of 850.degree. F. and an end boiling point in excess of 1200.degree. F. Generally speaking, heavy oils will have an initial boiling point in excess of 500.degree. F. and a 90% overhead point in excess of 1000.degree. F. Heavy gas oils and residuums are especially difficult to crack to valuable products because their boiling point makes satisfactory vaporization very difficult, their viscosity complicates handling and further complicates vaporization, the metal contaminant concentration for such oils is usually quite high, the hydrogen:carbon ratio is usually quite low and the concentration of carbon producing components such as polycyclic aromatics, asphaltenes and the like is usually very high. Feeds which contain components which have a boiling point in excess of 1050.degree. F.+ are generally considered to be very poor fluid catalytic cracking feeds due to poor conversion to gasoline and lighter components, high coke production and excessive temperature levels in the regenerator.
Heavy oils can be successfully cracked to desirable products where they have been vaporized prior to contact with the catalyst and the catalyst:oil ratio is carefully controlled. With conventional feeds, vaporization is achieved by radiant energy transfer from the hot cracking catalyst to the feed droplets. This type of vaporization mechanism is satisfactory for oils boiling below thermal cracking temperatures which commence at about 850.degree. F. For heavy oils, however, vaporization by radiant energy transfer is unsatisfactory due to the onset of thermal cracking and coke formation prior to complete vaporization of the liquid. Coke laydown is worsened where liquid oil strikes the hot catalyst particles. It would be clearly desirable to provide an apparatus and process to mitigate contact between not catalyst and liquid oil feed in a catalytic cracking unit.
Steam is frequently used to assist in dissociation of the oil feedstock. However, the use of steam in a catalytic cracking unit can quickly become quite expensive because of the amounts involved. The additional presence of steam apparently functions to lower the partial pressure of components thereby promoting vaporization of said components. Also, large amounts of steam deactivate the cracking catalyst and change conditions in the riser-reactor. It would be very desirable to avoid the use of large amounts of steam for feedstock dissociation in a cracking unit if possible.